Due to the great variety of possibilities of development in the course of different processes, such as grinding, crystallizing, condensing--to mention only a few of them--particle masses have correspondingly different size distribution characteristics. Thus, particles will be present in different granular sizes after passing grinding installations, whereas, for example, plastic materials, such as polyethylene terephthalate or polyvinylchlorid after crystallization or polymerization will be present in the form of granules, charged with abrasion or dust particles. The size of such granular particles varies from about 3 to about 5 mm in diameter, and from about 2 to about 5 mm in length. Polyamide granules can even be larger than that. However, in contrast to this, the fine dust particles are maximally sized up to 50 micrometers. Larger particles, sometimes also called "angel's hair"or "birds', nests", may occur in the shapes of disks, balls or spheres and may have a diameter of about 2 cm to about 10 cm. They will develop particularly in connection with pneumatic conveying. Granular material transported by conveying pipes, slides along the pipe with friction, it heats up, forms adhering fibers or foils, which keep on tearing off or coming off. The length of such fibers or small hairs is extremely different; each of them may be only fractions of a millimeter long, however, due to positive connection it may be connected to one-meter-long formed bodies or balls. If, in the case of particle mixtures obtained from grinding plants, particular importance is attached to a classification with great separating efficiency, this will not be of importance in the second case, in which one intends to separate the granules both from substantially smaller dust particles and from substantially larger ones.
By means of known air classifiers, such as they have been described in the DE-A1-34 25 101, the DE-A1-30 24 853 or the U.S. Pat. No. 2,913,109, it is possible to separate heavier particles from lighter ones with great separating efficiency. This is accomplished by having the air classification stream flow in a vortex-shaped manner in the separating chamber and/or by dividing it into a plurality of partial flows of the same volume. To improve the separating effect there are provided distributing plates with blades; the heavier particles are thrown outwardly, the lighter ones are carried along by a centripetal air flow. The EP-B1-237 641 shows a combination, wherein the brittle grinding material from a roller mill is first comminuted on a feed plate with attached impact elements, so that the share of fines obtained is laid bare and is removed during the following classification.
Such classifiers are designed to separate fines from coarse shares; however, a simultaneous separation of fine and substantially larger particles to obtain granules thus cleaned is not possible with this procedure. It is true that a separation into several grain size fractions is possible with the help of multi-component sifters, even with a great separating efficiency, but this will entail considerable constructional expenses.